The spectral sensitivity of solid-state imaging elements such as CCDs employed in digital cameras and VTR cameras runs from visible range to near-infrared range around 1,100 nm. Accordingly, filters absorbing light in near-infrared range are employed to obtain images approximating the visible sensitivity of humans. As glass for this purpose, glass obtained by adding CuO to phosphate glass has been employed as filter glass, but phosphate glass has drawbacks such as poor weatherability and a tendency to develop surface roughness and clouding when exposed to high temperature and high humidity for extended periods. Thus, near-infrared light-absorbing filter glass having a basic composition in the form of fluorophosphate glass comprising a fluorine component and having good weatherability is being developed and marketed.
For example, near-infrared light-absorbing filter glass in which CuO is added to fluorophosphate glass has been disclosed as such glass (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-204342).
With the size reduction in digital cameras and VTR cameras in recent years, a need has developed to save space in the optical systems of cameras. Thus, it has become desirable to reduce the thickness of near-infrared light-absorbing filter glass. However, when the thickness of conventional near-infrared light-absorbing filter glass is simply reduced, near-infrared light absorption decreases and the desired spectral characteristics are precluded. Thus, the quantity of coloring component must be increased to compensate for the reduced absorption caused by thickness reduction. However, increasing the concentration of copper in the above-described conventional near-infrared light-absorbing filter glass is problematic in that the copper valence changes, transmittance at near 400 nm decreases, and a blue-green coloration develops.
For example, in Example 2 of Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-204342, the transmittance for a glass thickness of 0.3 mm is given. In glasses other than glass containing As2O3, transmittance at 400 nm is much less than 80 percent. This indicates that when the glass thickness is reduced with a high concentration of copper, transmittance at 400 nm drops and a green coloration develops. In this Example, the deterioration in transmittance is corrected with As2O3. However, As2O3 is undesirable because it is a harmful component and it causes public harm when contained in sludge and waste liquid from polishing.
Further, when the amount of copper is increased, there are problems in that resistance to devitrification deteriorates, crystals tend to precipitate in glass, the liquid phase temperature increases, making it difficult to form glass, viscosity at the liquid phase temperature decreases, convection occurs in the melting glass in a formed glass, and striae tend to form.
In fluorophosphate glass, typifying near-infrared light-absorbing glass, the introduction of fluorine into phosphate glass improves weatherability. On the other hand, the introduction of fluorine also severs the network of phosphoric acid in the glass structure, resulting in reduced viscosity. There is also a drawback in that manufacturing is rendered difficult by the volatization of fluorine. In particular, in near-infrared light-absorbing glass obtained by adding copper to fluorophosphate glass, the greater the amount of copper added, the more unstable the glass becomes and the higher the liquid phase temperature becomes. Therefore, it also becomes necessary to employ a high forming temperature. When the forming temperature is set high, low-viscosity glass flows out, resulting in severe convection of glass in a mold and the development of sharp striae in the glass. Further, a high forming temperature results in the volatization of a large amount of fluorine, the fluorine concentration on the surface of the glass during forming decreases, a refractive index differential develops, and striae tend to develop in the surface. Further, when forming thick glass with large dimensions, the cooling rate was delayed, tending to result in devitrification (crystallization) of glass. For these reasons, fluorophosphate glass of high copper concentration has yielded unstable products, has produced extremely poor yields, and has been expensive, rendering mass production difficult.
As stated above, as imaging devices employing solid-state imaging elements have become widespread in recent years, the demand for filters for color compensating has increased. For example, filters for color compensating are installed in high-cost items such as high-end video cameras as well as relatively inexpensive items such as portable telephones with camera functions. Thus, the need has been increasing to supply a filter material in the form of glass having a near-infrared light-absorbing function with stable performance, in large quantities, and at low cost.
Copper-containing fluorophosphate glass is known as near-infrared light-absorbing glass (Japanese Examined Patent Publication (KOKOKU) Heisei No. 6-43254). However, in order to keep a high transmittance especially at wavelengths around 400 nm in the glass described in Japanese Examined Patent Publication (KOKOKU) Heisei No. 6-43254, it is required to use high-purity, optical glass-grade materials. Therefore, there is a problem in the form of high cost.
Further, in order to reduce the cost of glass, the cost reduction of starting materials by reducing the grade of the starting materials have been considered. However, the use of low-grade starting materials is problematic in that transmittance in the visible wavelength range, particularly transmittance at 400 nm, is decreased. When transmittance decreases even slightly at a wavelength of 400 nm, even by visual observation, the glass is observed to assume a deeper blue color, and thus it becomes difficult to conduct good color compensating. To eliminate such decreases in transmittance, it is conceivable that the thickness of the glass is reduced. However, for filters for color compensating, there is a requirement that color compensating be conducted with good balance over the entire wavelength range of sensitivity of the imaging element. When the thickness of the glass is reduced, good correction of sensitivity at other wavelengths is precluded.
Further, it is becoming impossible to respond to the current surge in demand for glass for color compensating in solid-state imaging elements simply by using high-purity glass starting materials. Accordingly, there is a need to reduce the grade of glass starting materials not simply to reduce cost, but also to meet the increasing demand for glass for color compensating.
The first object of the present invention is to provide near-infrared light-absorbing glass in which the drawbacks of such conventional near-infrared light-absorbing glass have been eliminated, in which good color compensating characteristics are maintained even without containing harmful arsenic, permitting the thinning of the glass, and having good weatherability and forming properties; a near-infrared light-absorbing element comprising such glass; a near-infrared light-absorbing filter employing such glass; and a method of manufacturing high-quality formed glass articles comprising such near-infrared light-absorbing glass.
The second object of the present invention is to provide, at low cost, near-infrared light-absorbing glass permitting good color compensating, a near-infrared light-absorbing element comprising such glass, and a near-infrared light-absorbing filter equipped with such elements.